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Do existing large commecial jets make use of differential thrust to control yaw?
Does existing Avionics / Control Systems on large Commercial Jets make use of differential thrust of engines much?
e.g. I was contemplating applications of this sort: Normally the thrust provided by each of multiple jet engines will be different due to difference in age, wear, build etc. and this is usually compensated by rudder trim.
But instead, would it be feasible for a controller to detect the slight yaw moment and modulate the throttle setting to compensate thereby negating the use of rudder trim?
What about a more general strategy in which the use of rudder trim, or more extremely rudder itself is reduced via automated differential thrust modulation of jet engines? i.e. Using differential thrust not just yaw compensation but yaw creation as well?
Do such systems already exist? Or is there a fundamental flaw in my reasoning that make these ideas not viable?

As far as I know, differential thrust is a last resort when hydraulics fail. See the A300 incident in Baghdad in 2003. If everything works, the control system does not use differential thrust. The pilot might do so manually when maneuvering on the ground, though.

Some yaw is compensated for with the Yaw Damper as well which is capable of actuating the rudder its self.

@PeterKämpf But why not do things the other way around: i.e. You differential thrust in preference to rudder / rudder trim? Wouldn't there be an efficiency advantage? Every time you use the rudder you add  drag, right?

@curious_cat: At small deflections the drag does not change. Only when the angle exceeds 10° would drag rise. Trimming small asymmetries with the rudder does not incur a drag penalty.

While there are no large electric commercial planes at the moment, it is likely that some of them will use differential thrust. Current iterations of Eviation Alice and Lilium jet are designed with this feature. The latter doesn't even have a vertical surface.

No, differential thrust is used in normal operations only when maneuvering on the ground, and then manually, not driven by a computerized control system.
It is preferable to run all engines at the same speed. Not only would you add additional frequencies, but also the beat frequency between pairs of two engines if speeds differ. This makes it harder to check with your ear that all engines are humming away nicely and generally creates more noise than when the engines run in sync. The answers to this question explain what the beat frequency is and how synchronization is achieved.
Next, the lag in engine response makes the control loop prone to oscillations. A rudder command is executed quickly and precisely, and when the deflections are below maybe 10°, no profile drag penalty is incurred. Induced drag due to lateral trim should be very small, especially at high speed. Trim deflections are normally much below that value, so an efficiency benefit from trimming with differential engine speeds is unlikely.
During take-off and initial climb both engines run at maximum thrust, and any trim requirement would reduce total thrust. This reduction is small, but sometimes every bit counts. During descent, the opposite happens and one engine might need to run at a higher thrust level than ideal. To decouple lateral trim and control from thrust makes sense in those cases.

Thanks @Peter Kämpf, this made things much clearer for me too. What remains a little difficult to grasp is that rudder deflections <~10 degrees incur no drag penalty. How can lift (sideways in this case) be created without drag? How should I compare this for instance with the drag caused by aileron deflections that lead to adverse yaw?

@RobVermeulen: Oh, right, induced drag is indeed created. Silly me - I forgot. The statement is only true for viscous drag.

@PeterKämpf Any quantification of how much the additional drag would be at various rudder deflections?


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What are the relative braking contributions from wheel brakes, spoilers and thrust reversers?
What is the relative contribution to braking forces in a typical large commercial aircraft  from the three different devices contributing to braking after touchdown:

Wheel brakes
Speed brakes / Spoilers
Reverse Thrusters

The relative contribution should change with velocity so even nicer if we have the velocity depended thrust contributions. e.g. Reverse Thrusters provide more braking at the initial high velocity part of the roll out.
I'm posting a illustrative (but totally made-up) graph to clarify for what I was looking for:

I am looking for either empirical data or theoretical predictions for the thrust. Qualitatively I'd expect the spoiler contribution to fall off with a square law dependence on velocity. I'd think the reverse thrust offers a somewhat constant component but perhaps higher initially due to parasitic drag by the clamshells? Not sure, and this is what I'm trying to get: A quantitative relationship.
In case the answer will vary hugely depending on aircraft, I'm open to any specific case being illustrated.
Let's use the limiting case of full brakes with anti skid on.
PS. Since spoilers directly add to drag as well as indirectly help braking by dumping lift teasing out their braking thrust contribution might be harder?

Good pilot braking technique should result in somewhat of a bell curve, though I can't speak for auto brake. The reverse thrust should as well. As you suggest, spoilers drag braking should vary according to the airspeed, and your post script is insightful. Can't add much more than that.

@JonathanWalters Thanks. Any idea about relative strengths? e.g.  I've used something like 40-40-20 at touchdown. Is that reasonable?

I really don't know. I do know that thrust reverse will not start immediately at touchdown, and I would think the same would be true with braking.

At touchdown you have negligible weight on wheels. Allow the aircraft to de-rotate first, so you have maybe 60% weight on wheels. Add spoilers, and the weight on wheels goes to maybe 80% after de-rotation.

Your question shows that you understand the issue already very well, and more detailed fractions of the single components depend on the particular airframe and engine, so my answer will not add much insight.
The only detail I can add is a plot of tire friction coefficient over speed. Unfortunately, I do not know the source of it; I collected it sometime in the past and never found a reason to doubt its validity.

Also, this document might be helpful. Below I copied Figure 3 from it, because it comes closest to answering your question. But keep in mind that the details depend on the particular aircraft and its systems, as well as runway condition, wind speed, tire and brake condition.

Typical deceleration forces during landing roll, from this document. Note that the aerodynamic drag with spoilers plus reverse thrust at high speed produce already more deceleration than what is desired at low auto brake setting.
In order to find the overall contribution of each braking component, compare the areas under their respective plot lines.

Thanks Peter! Very helpful information (as always)! One question: How about this thought experiment: Bring a landing aircraft to a dead stop  under three different conditions: Autobrakes only. Reversers only. And spoilers only. Any idea how the stopping distances would vary? A dead stop might be an unfairly stringent requirement for the reversers / spoilers so how about 20 knots.

Would the answer to my follow up question i.e. stopping distance ratios under different braking devices used individually be essentially the ratio of the three areas under the curves in your figure? If so, eyeballing the graphs, would you say it is a fair assumption that all three contribute roughly equal braking contributions?

Perhaps this deserves a separate question?

@curious_cat: No, the wheel brakes will dominate the picture. Note that the "Low Mode" setting allows for 3 kt/s deceleration, while there is a Medium Mode with 6 kt/s and a maximum deceleration with full brakes and 8 to 10 kt/s. Thrust reversers are retracted between 80 and 70 kts; if they were deployed to the standstill they would also contribute more. An experiment without drag will be hard to pull off, and spoilers alone will result in the longest stopping distance. There is a clear rank: 1. Wheel brakes, 2. Thrust reversers, 3. Spoilers.

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Does 3D Thrust vectoring (TVC) provide improved maneuverability over 2D TVC?
What are the additional capabilities that 3D thrust vectoring (TVC) provides over 2D thrust vectoring?

define "much better". in its current state this question is heavily opinion-based.

its more specifically in terms of maneuverability..

Thrust vectoring allows to control an aircraft when the airflow over its control surfaces has separated. Before thrust vectoring, the range of angles of attack in which an aircraft could be controlled was rather restricted.
With the X-31, it was for the first time possible to control a completely stalled aircraft, which enabled much quicker maneuvering in order to get the nose pointed at an adversary. This was powerfully demonstrated by mock air combats with an F-18. The X-31 used a rather simple arrangement with three paddles at the engine exhaust, and this allowed to create both pitch and yaw moments. This makes it what you call a 3D thrust vectoring aircraft.
Modern thrust vectoring nozzles are round and allow to deflect the thrust by up to 20° in both pitch and yaw, making them also what you call 3D. However, this geometry will scatter radar waves in all directions, which makes it unsuitable for stealth aircraft. Therefore, stealthy designs can use only two straight vanes above and below the jet exhaust, which will create only pitch moments. This is what you call 2D thrust vectoring. When two engines are installed, differential deflection will allow to create limited roll moments, but the powerful side forces which allowed the X-31 to rotate its nose (and radar) quickly into the path of its adversary will no longer be possible.
Stealthy thrust vectoring will still allow to control the aircraft at all angles of attack, but the yaw rates possible with a regular thrust vectoring nozzle are no longer achievable. This, and the structural mass of a rectangular exhaust, are the distinct disadvantages of stealthy nozzle designs.

You should state which is the 3D and which is the 2D variant.

@JanHudec: Done, but those terms are not really appropriate. They are more from pop culture than engineering.

Yes. But they are in the question, so they need to be mentioned.

In short, yes; the ability to vector thrust to the sides of the aircraft, inducing yaw, does provide an advantage in maneuvering over pitch-only vectoring.
Consider the Su-30 "SM" and "MK" variants, which have 3D thrust vectoring. Here's a Su-30 at an airshow (I'm unsure of the exact variant) performing a range of post-stall aerobatics:
https://www.youtube.com/watch?v=ujenrmoeALE
Many of these maneuvers require the pilot to be able to kick his rear end to one side or the other without the aid of aerodynamic control surfaces (such as the post-stall Immelmann, the J-turn, the Super Cobra which basically stands the jet on its tail in mid-air, etc) and thus could not be performed, at least not to the same degree, by an aircraft like the F-22. The utility of such maneuvers in aerial combat is unknown and probably low (standing still in mid-air is more likely to get you shot down than to provide any tactical advantage), which is one reason why full 3D vectoring hasn't been added to U.S. fighters (there was some talk about applying the systems seen on the F-15 ACTIVE and F-16 VISTA to combat planes, but nothing serious ever came of it, and most of those airframes are now slated for retirement).

An aircraft always needs "3D manoeuverability" (pitch, yaw, roll). Whether you use TVC to perform one, two or all three functions, exclusively, or in combination with aerodynamic control surfaces is a matter of design choices and optimization.
The choice truly depends on what are the requirements of the aircraft in question. Note that TVC in itself is not necessarily "better" than conventional control surfaces on the wing and/or tailplanes.

Aerodynamic control surfaces are very efficient (with enough speed), but take some space and induce loads in the airframe. Ailerons on a wing take space that could be used for flaps. Tailplanes are a pure cost of weight and drag that you pay for maneuverability (and stability).
TVC costs a lot of weight and complexity, but in turn has little to no direct penalty on drag. However, note that it has a rather limited efficiency (which is directly tied to the thrust of the aircraft -- which is not so good: ideally you want thrust and attitude control to be independent).

Maneuverability always has a cost. So the question really is: For a particular aircraft configuration, and for a particular requirement of maneuverability (in pitch, yaw, roll), and for a particular combination of optimization criteria (minimize weight, drag, costs, complexity,...): What is the combination of aerodynamic surfaces and TVC (and other means) that meets the requirement while minimizing the criteria?

(This does not answer the question) However, what is the added bonus of the third dimensionality of TVC over 2d? What is the combat use case for more maneuverability in yaw? Modern fighters change direction of flight by roll and pitch (roll until you can pull up into the new direction), not generally by yawing. This question is not TVC vs non-TVC, but 2d TVC vs 3d TVC.

@CGCampbell, I think this is just another way of looking at the question. There is always a need for some amount of yaw control, that might or might not be related to combat. My point is that whether you should implement it with 3D TVC (with its cost in weight & complexity) or with larger rudders (with their cost of drag) is a matter of optimization of the configuration.

Oh, I heartily agree with what you are saying. What I am saying is you did not answer the question(s) asked. Either the title, or the body... I can't up-vote you as is. (hint hint)

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Are there competitors to Jeppesen making Electronic Flight Folders?
Jeppesen has made Electronic Flight Folders and seems to be a pioneer in that. Do they have any competitors for that?

Can you explain a bit more about exactly what you mean by an EFF? In the GA world at least there are many tools available for flight planning and tracking - usually on a tablet - but I'm not sure if that's what you really mean.

For the UK NATS makes most of the charts which I frequently use for flight stimulation. I believe that for charts especially the Aerodrome charts are much better than the style of charts Jeppesen produce. The only downside is that the charts aren't compiled in the way that charts on Jeppesen are and must be compiled. But for outside the UK the best charts in my opinion are Jeppesen.

From what I see, yes, there is a company called Flightman which also makes EFFs. I don't see any other companies listed that make them.


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Why is induced drag less on a high span wing?
I have a basic question and sorry if it is a well known fact.
I understand that the induced drag is due to the tip vortices changing the effective angle of attack (downwash).
I searched some website and they mentioned that for high span wings the disturbance caused by tip vortices is less and hence, less induced drag.
Now the confusion arises.
As per the lifting line theory, the tip vortices strength will be as same as the bound vortex.
Does this mean that the induced drag is reduced only for a high span wing when having same bound vortex strength as a low span wing?
[For simplicity we can assume a elliptical lift distribution]

A high span wing of the same chord has more area than a low span wing, so it needs a smaller vortex for the same lift.

You start from wrong assumptions, which explains your doubts. The line

the induced drag is due to the tip vortices

is as true as saying that wet streets cause rain. Also, the opinion that the

tip vortices strength will be as same as the bound vortex

is wrong. Unfortunately, many authors don't understand the topic themselves and copy what others have written before without thinking the issue through. Ideally, you would forget about all what you have heard about vortices and lifting lines, but since you ask I will try to explain potential flow theory a little.
In potential flow theory, lift is caused by vortices which are caused by the movement of a wing through air. These vortices run along a closed line: Within the wing they form the bound vortex, then they leave the wing backwards as trailing vortices and are connected at the point where the movement started by the starting vortex.
Now comes the important part which most authors conveniently leave out: There is no single vortex; instead, potential flow assumes an infinite number of infinitesimally small vortices which form out of nowhere when lift is increased or speed is reduced. Consequently, no single vortex leaves the wing at the tips, instead, a sheet of vortices leaves the wing at the trailing edge. The change in strength of the bound vortices over span is equivalent to the strength of the vortices leaving the wing, so the vortices fade out towards the tips.
My advice is: If you do not want to operate or to write a potential flow code, do yourself a favor and forget all that. It is much better to interpret lift as the consequence of a pressure field around a wing which accelerates the air flowing around this wing downwards. Induced drag is simply the component of the resulting pressure forces parallel to the direction of movement, while the perpendicular component is lift. Please make sure to follow at least the last link; it gives a very good explanation what induced drag really is.
Tip vortices are the consequence of air filling the void above the downward moving air behind the wing. They are not originating from the wingtips, but the consequence of the vortex sheet rolling up (if you want to stay in that picture). Note that the distance between the cores of the vortices is much smaller than the wingspan. For an elliptic wing of span $b$, it is actually only $\frac{\pi}{4}\cdot b$
A higher wingspan allows to capture more air for lift creation, so less downward acceleration is needed. Lower downwash speed also causes a less powerful trailing vortex. Note that the mass of air affected by the wing grows with the square of the wingspan!

Thanks for the comments and I understand it. But it raises one more question. Since the vortex sheet will roll out into two tip vortices, how come a high span wing has lower vortex strength? Is it due to lower strength of smaller vortices (in sheet) caused by smaller change in span wise lift distribution?
In other words, is it due to lower loading of the wing? That is a wing with span L will be loaded less than the L - dL span wing for same weight.

@Selva: A wing of higher span but the same area will have a smaller chord and consequently a reduced vortex strength at the same angle of attack. Wing loading (lift/area) will be the same, but vortex strength is proportional to chord length and has more span to act upon, so the same lift is produced by a weaker vortex.

Ah.. Now I get it. Thanks a lot for your answers.

Your answers, as always, are impeccable.

@Peter Kämpf, Peter, I seriously think you should write some of the ATPL books for pilots AND examiners (at least Principles of flight) since even the most popular ones (if not all) explain things in very illogical manner and contradicting common sence. Tip vortices is just really good example. Thanks.

So, even this NASA article describes wingtip drag as induced drag https://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-15-DFRC.html Quote: "Winglets increase an aircraft's operating efficiency by reducing what is called induced drag at the tips of the wings." Is there a disagreement about what is and is not induced drag? See also https://www.grc.nasa.gov/WWW/K-12/airplane/induced.html

@atlex2: You shouldn't believe something that was probably hacked together by an intern. Just because it has a NASA URL doesn't mean it is the holy truth. Better judge for yourself whether that explanation makes sense and does not violate the laws of physics. The picture in the second link is plain wrong.

@PeterKämpf What makes sense is that the energy that goes into making a wake vortex needs to come from somewhere, and accounting for it as part of drag makes the most sense. So far, people seem to call that induced drag. I'm still really scratching my head here, but you're right to say that an intern probably created that website.

If I may ask two things: 1) bringing your explanation about the rearward component of the total aerodynamic force to an extreme: imagine a door flying through air at 90 deg AoA; from your explanation, the total aerodynamic force formed by pressure difference on both sides will be facing backwards and thus in your terms fully equal to “induced drag”. But there’s no lift produced, so this “induced drag” explanation contradicts the common wisdom that induced drag only exists where lift is produced.

(Breaking in two) 2) How would you comment the Denker’s approach to explaining that (note he is careful with formulations onwhat is cause and what is consequence, saying “It doesn’t matter whether you consider the vorticity to be the cause or the effect of the descending air — you can’t have one without the other.”) https://www.av8n.com/how/htm/airfoils.html#sec-circulation His formulation is attractive because it operates in energy terms, saying that if certain energy was lost into tip vortices, there must exist a force that did the work.

@agronskiy: Induced drag is pressure drag, but not all pressure drag is induced drag. Denker is right when he says you cannot have descending air without vorticity, but to say that it doesn't matter which causes what is not rigorous enough. Clearly, air is accelerated downwards (that is where the energy is spent!) and that downward traveling air causes vorticity when the surrounding air fills up the space above it. Vorticity and friction is how that extra energy is dissipated eventually.

Thx, I see! After reading tons of materials on that I start feeling that it is in the end slightly underdefined terminology, coming from a shady, vague border between pressure drag and induced drag: i.e. in my example about the door — where this pressure drag stops being “induced”? With loss of lift? But this division is somewhat arbitrary... I appreciate you pointing out that the energy is lost from downward acceleration (i.e. even by infinite wing), but this can be attributed to pressure drag, isn’t it?

@agronskiy: Yes, there are tons of material out there, but quality varies. Sometimes a lot. Now about the door: If no net down- (or up-)ward deflection of the air happens past the door, there is no induced drag. But still a lot of pressure drag. Please keep in mind that induced drag got its name from electrical induction: The Biot-Savart law allowed to calculate lift-dependent drag for the first time; quite some revelation in its days. But in the end the definition of induced drag is arbitrary; I prefer to see only friction (parallel) and pressure (normal) drag components. All further (cont.)

@agronskiy: subdivision only makes things overly complicated. With more drag components you need to avoid double- or undercounting, so dividing drag into pressure and induced drag leads to more confusion, as you found out. Again: Ideally, you would forget about all what you have heard about vortices and lifting lines.

Thanks a lot for your clarifications which offer some fresh view on the whole thing!

I have one question about wingspan effect on reducing induced drag - will increasing wing chord without change in wingspan also reduce induced drag? Because as well as increasing span, increasing chord will give greater wing area, so less angle of attack is needed to maintain required lift and so lower air down acceleration needed. Correct me if I’’ wrong please.

@Konrad: Increasing chord means to increase reference area, too. The induced drag coefficient becomes smaller, but the induced drag force will be the same (plus some more friction drag). See here for a longer explanation.

@PeterKämpf But increasing span doesn't increase reference area too?

How I understand it, correct me if I'm wrong: induced drag will be lower if we accelerate more air apart from accelerating same amount of air more. Increasing wingspan allows to accelerate more air, and increasing chord will not change the amount of air accelerated, it will only increase the acceleration i.e. due to camber. And will induced drag be lower if we increase wingspan but we also reduce wing chord (if compared to increasing wingspan and staying with the same chord)?

@PeterKämpf "induced drag is simply the component of the resulting pressure forces parallel to the direction of movement" But 2D airfoil also has  pressure vector point back  when do integration of static pressure?  https://aviation.stackexchange.com/questions/98827/does-pressure-vector-point-back-at-2d-airfoil-in-viscid-flow

@user207141 This line is only true for inviscid flow. I was sloppy. Friction adds mostly shear drag but also changes the pressure distribution such that some pressure drag is added, too.

@PeterKämpf So induced drag is only drag that exist in inviscid flow. When I integrate static pressure around real wing(from static pressure taps), how  we call  this drag?

@user207141 It is a mixture of induced and pressure drag, depending on your reference. Is x the airfoil chord? Or the direction of movement? Reality is messy.

A simple way to understand this which I've never seen anywhere is to consider the air pushed down by the wing as the aircraft passes. If aircraft 1 has a wider wingspan than aircraft 2 (but the same wing area), then aircraft 1 accelerates a wider swath of air downwards than aircraft 2. Lift is proportional to the momentum transferred to the air being pushed downwards, while the energy required to do the downwards pushing is proportional to the square of the downward velocity. Pushing down on the wider swath of air means the air doesn't have to move as fast as the smaller swath to give the same lift. Since momentum is the same for both the large and small swath, the energy imparted to the larger (more massive) swath of air is smaller because of the velocity-squared term.
This offers an intuitive explanation for why induced drag drops as speed rises: at low speeds the mass of air pushed downward is proportional to the wingspan times the distance covered per second. At higher speeds the distance covered per second rises, giving the aircraft more mass to push against. Double the air mass by doubling the speed and the downward air velocity drops by half, which cuts the energy required by a factor of root two.
This extends to explain why an infinitely long wing produces zero induced drag: the accelerated air mass is infinite, so the air velocity imparted by downwards acceleration is zero, which costs no energy.
In a real aircraft the air doesn't go straight down, so you get vortices. But the momentum/energy relationship holds, as does the main principle: the larger the mass of air, the less you have to accelerate it downward to get the upward lift you want.

so the air velocity imparted by downwards acceleration is zero afaik, not true. source?

@Federico: This is for an infinite wing only. And true.

If you read @JanHudec's and my answers, you will find plenty of mention of your view of lift creation. +1

@PeterKämpf I noted the "infinite wing", I still don't see how it is true from the equations. I reiterate my request for a source.

@Federico: Consider an infinite wing of constant angle of attack, airfoil and chord. All bound vortices remain in the wing, there is no free vortex leaving it and, consequently, no downwash. This is really only a thought experiment and, if followed through, means that lift is also zero.

@PeterKämpf ah, that's what was confusing me. by reading it I understood that the answer claimed that an infinite wing would produce lift with no downwash. ok, if you say that there is also no lift I get it.

@Federico: The infinite wing will produce infinite lift with the slightest circulation. The easy solution is to let it produce no lift by giving it an infinitesimally small chord.

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Are there any aircraft that can engage thrust reversers above idle power setting?
Based on my experience with jet transport aircraft, thrust reverser levers can only be engaged after reducing the throttle to idle. Do any jet-powered aircraft exist that can engage the levers at power settings above idle?

Define thrust reverser.

@Jim Thrust reverser is a mechanism that changes the direction of the thrust vector by approximately 180°, while preserving as much of it's magnitude as possible.

More like 100-120 deg. The deceleration you get is fairly modest, and is not even included in stopping distance performance data. Basically considered a bonus. Almost all of your stopping power is brakes. Cascade style reversers are the worst.

Most thrust reversers divert the flow about 90 degrees.  Rather than reverse thrust, this eliminates the jet thrust -- leaving the rest of the engine as a tremendous drag producer.  This reduces landing distance.  They are omitted from performance data because you must still be able to land if the reversers fail.  A few aircraft (including the C-17) turn the flow enough to truly reverse thrust.

@AdityaSharma - Yes. I was probing to see whether their definition covered things like thrust vectoring.

Are you only interested in "as delivered" or also in "after modification"?

Probable duplicate of https://aviation.stackexchange.com/q/3660/7532  I don't want to start the close-vote on it, because (as a mod) my action would immediately close it, and this is a reasonable vote for the community.  But if somebody would nominate the question to be closed-as-duplicate, that would be great.

@RobMcDonald doesn't the engine still suck air in, creating forward thrust?

@Someone Thrust is $F_n=\dot{m}(V_j-V_i)$ proportional to the change in velocity through the engine (axial component).  If the aircraft is moving forward, the $V_i$ will be similar to the forward speed.  If the thrust reversers turn the flow (essentially bringing the axial component of speed to be less than $V_i$, then the $\Delta V$ term will be negative and you will produce a negative force.

There are no commercial aviation aircraft that are made this way. There are physical locks inside the throttle levers that make it impossible to raise the reverse thrust lever if the throttle lever is forward of idle.
Now, you say "any"...that's pretty broad. I'm not going to say there's never been some obscure, weird airplane no one's ever heard of that might have been designed like you're saying, but it's hard to imagine why any manufacturer, civilian or military, would think this would be a good idea.
Edit:

The source is my own experience. I am personally familiar with a wide variety of commercial jet aircraft spanning several decades of flying them, and no manufacturer designs TRs like this. Just the opposite: just having a TR become unlocked when it isn't supposed to is a problem that requires immediate attention. If the TR actually deploys, the engine will be commanded to idle by the EEC; in some older designs before EECs, the throttle would actually slam back to idle.

This question was already answered here almost 9 years ago anyway.

Nice find on the duplicate question!

"some obscure, weird airplane" -- The heavily modified biz jet used by astronauts to practice landing the space shuttle needed a lot of drag in order to be appropriately brick-lick.  I think it had the ability to deploy thrust reversers in flight.  But that is certainly an obscure, weird airplane.

The C-17's reversers can be used to back up on the ground.  The pilot must throttle-up to do this.
They can also be deployed in flight during a tactical descent.  I believe the throttles are usually kept at idle during this maneuver.

@757toga do they need to spool down to flight idle before engaging the thrust reverser mechanism?

@757toga sure, thanks

This doesn't answer the question! There are many aircraft that can use reversers to back up on the ground as well as in the air. However, in all of those I'm familiar with, you first have to retard the thrust levers to idle, then engage the reversers, and then you can add thrust again (usually with the reverse levers).

@ Bianfable - any aircraft design that proposed allowing a pilot to engage thrust reversers without first going to zero throttle would immediately fail a host of FHAs, PSSAs and FMEAs. i.e. it'd be a fundamentally unsafe design which the regulatory authorities would correctly demand be amended.

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Which engine is more efficient between turboprop vs jet?
Are turboprops really more efficient than jets?
If yes, does that mean that airlines who use jets sacrifice fuel for faster flights?

The short answer to both your questions are Yes. The long answer is, it's not so simple.
The ATR 72 you have pictured has a top speed of 276 knots and a range of 1,300 miles. It can carry around 70 passengers.
This document provides detailed comparisons of burn rates. So we can see the ATR 72 burns about 810 Liters per hour (about 214 gallons/hr). A Boeing 737-400 seats about 150, has a maximum range of about 2,400 miles, and a cruising speed of about 473 knots (almost double the range, and over 1.5 times the speed). But its burn rate is about 3,000 liters per hour (793 gallons).
So is it a strict trade-off? Speed for Efficiency? Well, not entirely.
TAS is true airspeed. That is, the speed of the aircraft relative to the airmass in which it is flying. However, an decrease in density (e.g. high altitudes; air is less dense) will yield an increase in TAS. Thus, it is easier to fly at the same true airspeed at higher altitudes. Since jets are generally used on longer flights (where much of the time in the air is spent at high cruise altitude), they have to work less to fly the same speed as they would 10,000 feet below. There are other reasons why jets are more efficient at high altitudes - they have to combine less fuel with the thinner air, and there is less drag at high altitudes.
So why don't propeller planes fly at high altitudes then? Well, part of it is that turboprops are less efficient at higher speeds and higher altitudes, and jets (turbofans) are more efficient at both (obviously to a point, but airliners don't fly fast or high enough to decrease their efficiency again).
Since jet engines can also produce more thrust, they are better suited to large airplanes, which are the ones used for long distances (e.g. if you wanted a propeller plane the size of an Airbus A380, you would need a lot more than 4 engines). So jets are used in situations where they are more economical, and props where they are more economical.
In summary:
Turboprop characteristics

Efficient at low speed, low altitude
Low thrust

This leads to smaller aircraft on shorter, slower routes, like you've observed, since they don't have to climb or fly fast to get there in a reasonable amount of time. Also these short hops generally require fewer passengers, or flying a second flight is better than needing a much bigger plane.
Turbofan characteristics

Efficient at high speed, high altitude
High thrust

This leads to larger aircraft on longer, faster routes, like you've observed, since they need to fly higher and faster to get there in a reasonable amount of time. Also since these routes are more "popular", and it costs so much to fly that far, it makes more sense to put more passengers on one plane, than two.
In general, an airline would not sacrifice fuel for faster flights if that was the only advantage, unless they knew there could be a price premium for the faster flight (e.g. Concorde).

I don't understands your summary, you say that turboprops are both low and high trust, and are efficient slow and fast, high and low

TAS has no dependence on air density.

For short distances, turboprops are more fuel efficient than jets. On the longer flights, the jet (turbofan) engines win the race.
Turbofan engines perform best at high altitudes. At medium and low altitudes, the turboprops are more efficient engines. Since all flights start and end at low altitudes, the high altitude cruise segment needs to be long enough for the turbofan engine to offset it's disadvantage during start and landing.
The break even point is somewhere between 300 and 500 NM.
The jets are indeed faster, but on short flights the time difference is marginal. A large percentage of the time is spent on the airport and at low altitude where speed is often restricted. The longer the flight is distance wise, the time factor becomes an import advantage for the jet. Not only do passenger like the shorter flight, but it also means less cost for the operator that pays pilots and flight attendants by the hour.

So this is why "Austrian" prefers Turboprop Aircraft for domestic flights, because Austria is so small

Yes, that is probably the reason. Another example is FlyBe that operates the Dash 8 on short sectors from Southampton, and the Embraer jets mostly on longer flights.

The cost for the pilot and staff is fairly minimal as far as a time difference. The fuel being saved is the bigger factor here.

If the increased speed means the plane can fly another flight per day, that's a significant increase in revenue.  Some of the costs associated with owning and operating a plane don't increase with how much they fly.

@TaherElhouderi i think the longest route is Vienna-Altenrhein which is 512km.

So I don't understand why airlines don't use turboprop for the SDU-CGH route (366 km), insead using 737s.


text:
What if you don't recognise an aircraft ATC is telling you about?
Often an ATS will warn you about other aircraft: when you're taxying and need to give way, or for land-after (and other conditional) clearances, or to warn of traffic in the area you're flying in. Usually they do this by naming the type of the traffic, and often you need to positively confirm whether you have the traffic in sight. For example:-

"G-ABCD, report departing Squirrel in sight"
  "Affirm, traffic in sight, G-ABCD."
  "G-ABCD, behind the Squirrel, cross runway 18 behind, report vacated."
  "Behind the Squirrel, cross runway 18, wilco, G-ABCD."

What if the type of aircraft you're warned about is one you don't know? In an area with lots of vintage or experimental aircraft, you might not even know whether to look for a fixed-wing or a helo. I wouldn't want to say I have the traffic in sight if there's a risk I'm not looking at the same aircraft.

"G-ABCD, report departing Squirrel in sight"
  "The what? Is there wildlife on the runway? G-ABCD"

Is there a standard way to explain the situation and disambiguate? Failing that, is there a concise way that will not be misunderstood over noisy radios?

(For the benefit of other readers who wouldn't recognise a Squirrel: it's a helicopter, and the local police operate one out of Cambridge. It was the first obscure-sounding aircraft name I could think of to use as an example.)

ATC must have been talking about this squirrel.

I think "Unfamiliar with type Squirrel, request more details, G-ABCD" would make the point

"G-ABCD do you have the...wait, what is that thing!? I don't know how to describe it...just follow it, whatever it is..."

Cambridge, U.S. or Cambridge, UK? If it's the UK, the police operate EC135s...

To add to Dave's answer, if traffic is pointed out to you which you are unfamiliar with, ask for clarification. ATC or FIS will have a good understanding of aircraft types and speeds, especially in radar environments the speed will be available as groundspeed (GS).

FIS: DELFF, traffic information, 3 o'clock, 4 miles, same altitude, Mooney Bravo, passing right to left.
  PIC: DELFF, unfamiliar with aircraft type.
  FIS: DELFF, traffic is a low wing aircraft, 100kt groundspeed

It might be worth noting that the clarification might take a few seconds; if you're unfamiliar with the type, the ATC can't be expected to know the configuration off the top of his head; if he doesn't have the plane in sight, he might have to consult a spotter's manual. While there are relatively few configurations of body, wing, tail, engines etc, there are thousands of makes and models of GA craft in operation to say nothing of military and commercial designs.

While I understand what you worry about this is a skill that comes with both time and experience. For what it's worth it's far more important that you know there is an aircraft there than you can tell its make and model. As you fly more you will learn the common ones just by seeing them around the airport and hearing other pilots talk about them. There will always be situations where they may call out an aircraft you don't know but I would not ask for clarification if you have the aircraft in sight. Another reason to call out the make and model is for speed reasons. If you are flying say a Mooney (a fast little plane) on down wind and there is a Piper Cub ahead of you turning base now you are going to want to extend your downwind to give him space (although the tower may call this for you anyway).
Now let's say you hear the controller call:

Number 2 behind Blohm & Voss BV turning base now, cleared to landing runway 24

If you can see a plane turning base you can respond:

Traffic in sight, #2 cleared to land runway 24

If you can find a plane where the controller says it should be I would think you are OK. That being said if there are 2 planes, and you only see one you need to report what you see. If you are not sure which unknown model it is, report the location, not the make and model.

I would simply describe the plane I see like.  "I have the low-wing aircraft/jet/whatever in sight"  If it's not the right one hopefully they'll catch it and elaborate.


text:
Can an A&P who is also a private pilot receive compensation for conducting maintenance test flights?
Related to this question.
Suppose a repair is made to an airplane and a maintenance test flight is required. Suppose the A&P who works on the aircraft is also a private pilot. That person performs the test flight and then signs off the repair.  (assume USA)
A private pilot can't fly for compensation or hire.
Can the mechanic bill for the time she spent flying?  Are maintenance test flights considered incidental to a mechanic's business?

Even better, does the mechanic have to pay for the fuel burned during the flight?

Yes, the mechanic can fly the aircraft, according to this FAA legal interpretation (Williams, 2014):

The scenario you present is a mechanic who, as part of his duties,
  conducts flight tests after performing maintenance on the aircraft,
  and does not carry passengers or property for compensation or hire.
  The mechanic holds a private pilot certificate with a valid third
  class medical certificate. This situation is covered under §
  61.113(b), which states that, "[a] private pilot may, for compensation or hire, act as pilot in command of an aircraft in connection with any
  business or employment if (1) the flight is only incidental to that
  business or employment and (2) the aircraft does not carry passengers
  or property for compensation or hire." As these flight tests are
  incidental to his employment as a mechanic, the mechanic is not in
  violation of § 61.113 when conducting these flight tests

Well how about that.


text:
Should full flaps be deployed on takeoff?
Are full flaps ever used on takeoff?
One flying book I read strongly discouraged anything more than quarter flaps on most planes due to the amount of drag produced. I was just wondering if there are any scenarios where full flaps might be necessary.

Even on a go-around taking the last notch out is generally recommended, the amount of drag goes up a ton for only a little bit of extra lift on the last setting usually.

@p1l0t Tell me about it. Going around with full flaps is not a pretty sight. Tower even asked me if I had engine trouble.

The POH is authoritative source for the valid flap takeoff configurations.

I did that once (full flap touch and go).  It wasn't good.  The aircraft felt super sluggish, like I was in slow flight while taking off... Definitely not something you want when you are that close to the ground.

First, you have to remember on some configurations, the lower stages of flap will mostly add drag and very little lift. Sometimes that extra notch of full flaps is only there to change the camber of the wing to add a bit of a nose down attitude to help with visibility on landing. Both are items you do not require on takeoff.
Now, when selecting flaps for take-off there are two things to consider: runway length and obstacles to clear after the runway:
Runway Length
Generally when lowering flaps your $V_{\text{mu}}$ (the speed at which you become airborne, “minimum unstick speed”) will decrease. This  means a shorter runway for normal stages of flaps. As you continue to add more flaps your $V_{\text{mu}}$ will be lower, but because of the added drag, which decreases acceleration,  it will actually take a longer distance to reach that lower $V_{\text{mu}}$. There is a sweet spot in-between, though. “Take-off Flaps” is not always designed to give you the shortest takeoff roll.
Obstacle Clearance
And that is because once airborne, the more flaps you have deployed, the higher the drag, the less excess thrust available, thus the less you can climb. Simply put, you are now flying at a low speed and barely climbing. Not a good idea if there are tall trees waiting for you at the end of the runway.
It would be illegal  to take-off on full flaps if:

POH says it is prohibited,or
For commercial flying, if the flap setting does not provide the minimum required gross obstacle clearance climb gradient (depending on the type of operation and number of engines, between 2.4%-3.0% ).

Then there are some crazy bush flying techniques like accelerating w/out flaps to Vunstick, lowering flaps to bubble off the runway, accelerate in ground effect while retracting the flaps, and climbing at Vx. Guaranteed to scare your instructor and/or fail you at your checkride!!

wow, that's pretty crazy. also, is there a 'correct' term for Vunstick??

Actually Vmu is the correct term, with mu meaning 'minimum unstick'. Can't find a decent page on the internet about it though.

Ah, ok, I thought 'unstick' was an informal term ;-)

I really like this answer because it provides a lot of useful information, but I think the first paragraph has two wrong statements. (1) remember that in a wing, the induced drag is induced by lift, so lift must be increased in order for drag to increase (see http://www.vatnz.net/_media/images/raw/image3.JPG) (2) That last notch of flaps is there to increase the angle of descent, not to provide visibility. I wasn't sure  how to edit the first paragraph without changing the whole answer.

edited the text for (1), which is true. For (2) I'd still argue with a list of (small) a/c that have the landing flaps there to compensate poor fwd visibility from the cockpit.

@rbp, I've vetoed your suggested change: As a 1.70m pilot I really struggle with fwd visibility for flaps-up landings on some aircraft. Full flaps really helps in all Diamonds a/c and on some Cessnas.

@Radu094 just because you personally get better forward visibility, that doesn't mean that full flaps were designed to improve visibility. If you can find any actual evidence that any aircraft manufacturer added landing flaps to improve visibility rather than improve the angle of approach, then I will retract my objection.

Typically more flaps will get you in the air sooner, but due to the added drag your climb will be shallower. So when the runway is really short and there are no obstacles, full flaps might be the solution.
There are aircraft that are not certified for full flap take-offs and in some cases that additional drag might in fact reduce the performance so badly that the take-off run will be extended. The operating manual will provide guidance.

the standard procedure for a short-field takeoff is to lift off into ground effect, accelerate the aircraft in ground effect until reaching Vx, then pitching the nose for Vx. Full flaps will increase the induced drag as the plane accelerates, thereby increasing the runway length required.

Not all flaps are the same and you should read the poh for the suggested and prescribed way to operate the aircraft you are flying.
For example the Grumman AA5 series flaps provide a lot of drag and no real extra lift nor slow speed ability.  They do allow sharp descents.
Whereas a Cessna 172 flaps will increase lift and lower stall speed.
Read the manuals.

Full flaps can be used on take-off whenever it is called for in the POH, but I have never seen it in any of the 40 different aircraft I have flown.
In some aircraft that I've flown (eg Cherokees and the Pilatus PC-12), there is an intermediate flap settings (about 25 degrees) which is used for short/soft field takeoffs because the extra lift from the downwash from the flaps helps the plane liftoff.

As a certification requirement, all planes must be able to climb from sea level at max gross weight with full flaps deployed, in standard day conditions.  As has been noted by others, the rate of ciimb will be anemic.
In the PA-28 that I fly, the third notch of flaps adds a lot of drag and very little lift.  POH procedure is to use two notches, not three, of flaps for a short field takeoff.

Are you sure about the certification part? I would be very surprised if a fully loaded 160HP C172 with full flaps would climb at all.

Straight out of the factory it had to, or it wouldn't get certified.  But engines do get a bit tired with years of use and abuse.  Certification requirements are a one time pass or fail.  Then, as long as the planes are made under the same type certificate, they do not need to pass the certification requirements again.

@SkipMiller: not necessarily -- if the aircraft can retract the flaps (partially or fully) in 2 seconds or less, it can use that flap setting for the go-around (14 CFR §23.77).

What year was this in force?

Please cite a reference for this certification requirement. My airplane (M20M) will barely climb at sea level with full flaps and 36", let alone to 25,000ft (its certified ceiling).

The takeoff performance requirements in FAR Part 23 are predicated on the use of the takeoff flaps setting.


text:
How are pushbacks coordinated between the pilot and the tug?
When the pilot (PIC) of a large jet is cleared for pushback and start, how does he signal the tug to start towing the aircraft, and where to go to?
The pilot has to monitor the ground frequency the whole time, so is there a second radio for talking to the tug crew? Or perhaps a wired link between the aircraft and the tug?
Does he just tell them to "take us to start point alpha" or whatever, or does he have to give more detailed instructions?
Are the tug crew listening in to the instructions from the tower?
If the aircraft has to stop unexpectedly (because of an instruction from the tower, or because the pilot has a safety concern), how is this coordinated safely?

Nice animation: Aircraft Marshalling and Push back Procedure on YouTube.

Generally speaking1, when an aircraft is allowed/ready to leave (deliberately not using the word depart) the gate, things proceed in the following fashion:

The pilots are in contact with Ground Control, and when the aircraft is ready to leave, Ground approves that they can leave the gate.
The pilots asks groundcrew that the pushback can start.
Pilots are in contact with a designated personal from groundcrew (see picture below).
Pushback tractor/tug will start moving the aircraft away from gates, onto the place where aircraft will be ready to move on its own.
During the pushback, aircraft's engines are started2.
Pilots inform tug driver/designated person from groundcrew which direction the aircraft needs to be facing at the conclusion of pushback.
When aircraft is ready to roll towards its assigned taxi instructions, marshaller signals the pilots that aircraft is in the requested position and pushback tug is about to be detached.
After the tug is disconnected, the designated groundcrew personal communicating with pilots, informs the pilots and disconnects his headset. The marshaller signals (after moving away from the aircraft) the pilots that the aircraft is ready to proceed on its taxi route.
Pilots confirm and acknowledge this last signal, and aircraft starts to taxi.

I have read that different airports have different procedures for the above mentioned activities. Although, I have always seen that a person from groundcrew attaches a headset near the nose gear of the aircraft (picture below) to communicate with the pilot.

Image Source
Please see the cable connecting the headset with the aircraft, near the nose gear. You cannot follow the entire cable though.

1: This is what a typical scenario looks like. Although, there can be great variations.
2: Engines are not started near the gate as there is groundcrew servicing the airplane and to avoid damages to close-by structures.

So the designated ground crew person walk next to the plane as it's being pushed back?

@raptortech97 they also keep a lookout for wing tip clearance

Yes, there are many different variations on the above. Also, the ground crew typically informs the pilot when it is safe to start the engines (which may actually be near the gate). There are also hand signals when electronic communcations are not available/functioning.

Does the ground crew also shout "contact" like when I start my Tiger Moth? :-)

It does look weird, some guy/gal with a plane on the end of a cable. like they're taking a 200-tonne dog for exercise.  I always want to shout 'Walkies' :)

Just as a note, some very busy airports (Tokyo Narita is one) have a separate ramp control frequency that you contact for pushback. Then when you're pointed in the right direction, engines running, tug disconnected, they hand you off to ground control. Also, there are often two ramp/ground control frequencies depending on where you are on the airport.

I used to initiate contact with the marshaller by variations on saying, "Cockpit to ground, are you there?" or just "Cockpit here, are you plugged in?" However, I flew for a time with a very gregarious captain who made a point of getting to know the ground crew well in situations where we had the same ground crew night after night. He would initiate contact by saying, "Sky god to ground scum."

@DanHulme - I thought the term nowadays was "Clear prop!".

@KeithS If you clear the prop area, who is going to swing the prop?


text:
What are the limits to lateral Centre of Gravity imbalances?
I've never seen lateral CG limits prescribed for aircraft (though I have very imited experience in this realm).  Presumably, aircraft are designed to keep the center of balance as close as possible to center of lift, with any imbalances are far more likely to occur fore to aft than side to side.
But, for instance, what if fuel only drained from one wing during flight?  How much of an imbalance would this create?
Are there on-board systems in general aviation to prevent this?
What are the effects of a lateral imbalance on handling, and how could pilots compensate, if possible?
Edit 1: Please do not restrict your answers to fuel loads.  Is there a percentage-based (e.g. greater than X % aircraft total wingspan) or positional-based (e.g. lateral CG moved outside of the fuselage) DNE from an engineering standpoint?
Edit 2:
Also seeking design input, considering experimental and purpose-built aircraft such as White Knight Two exist.
I also note that despite the larger cargo volume of the Airbus Beluga, it cannot carry near as much weight as the smaller-volume C-5 Galaxy.  Is this a benefit of the C-5's design in placing the wings above the fuselage or is there such a thing as vertical CG limits?

On the Beluga -- what you're likely seeing is a structural weight limit -- vertical CG isn't an issue on any aircraft I know of

I assume that you are only interested in fixed wing?  You might consider editing the question to make this explicit.

nope, all answers welcome.  speaking of rotary wing (especially the cargo variety), i'm curious how sling-loads play into the equation.

Helicopters have quite narrow lateral cg limits, and they are different for left and right. If you want to know more, ask a new question. The comments are not suitable for answers.

i see my original question was too broad.  i'll continue prodding in a separate question.

Lateral c.g. limits are prescribed for large aircraft both for max fuel imbalance and for what is being carried in the fuselage. It's basically impossible to exceed the max lateral imbalance in a passenger aircraft. Exceeding the max in wide body cargo aircraft can be done and is checked for, but it takes an unusual load to exceed the max.
The 747-400, for example, has a max lateral imbalance of 10,000,000 inch-pounds. If you load 207,205 lbs on one side only of the main deck of a 747-400 freighter using size code M pallets, you will exceed the maximum. To see this happen (if you're using Firefox or Chrome--IE won't work) go to 747.terryliittschwager.com, dismiss the flash screen, and then click on test load #6 near the bottom of the page. That will trigger exceeding the lateral imbalance limit. Scroll down to the Lateral Load Imbalance graphic to see the max lateral loading envelope and the point that shows the load exceeding it.
Here are some approximate weights for a 747-400 freighter:
basic operating weight 365,000 lbs
max fuel 360,000 lbs
max cargo 245,000 lbs given the 610,000 max zero fuel limitation and a 365,000 bow
Note that you cannot carry max fuel and max cargo. A common 747-400 max takeoff weight is 875,000 lbs.

great site!  i see where your lateral imbalance example exceeds the envelope.  what units are the x-axis values in?

@erich Millions of inch pounds. Eventually I'll get the axes properly labeled (if I ever finish the project). The axis labeling will a bit tedious since if the user makes the weight unit kilograms, that axis labeling will have to become inch-kilograms, which is a real bastardized unit in my opinion, but that's how Boeing shows it for kilograms.

Millions of inch pounds, well, that's ridiculous. On the other hand, we should be probably happy that it's not a dozen^5 inch-pounds, right? /sarcasm

Calculator you linked will work in IE if you will allow it to ;) It is set to (for no good reason) work in emulated IE8, simply setting it to 9 or higher will make it work.

@terry agree on the bastardized unit.  in that case i'd also change the arm units to meters in that case as well and go full SI (also using Nm for moments as well).  on a related note, do any manufacturers supply W&B data in metric as standard?

Nifty looking page, @Terry! If I had any clue what it all meant, I'd be most impressed. Maybe I should go to flight school to understand what y'all are talking about... :/

@FreeMan Flight school might not help as pilots rarely get involved in the detail of properly loading large cargo aircraft. Boeing offers (at least they used to) a one-week course for loadmasters on Boeing freighters.

@yo' The U.S. Metric Board was established and funded in 1975 to convert the U.S. to the metric system. One of the many sins of the Reagan administration was bowing to conservative pressure to defund and then abolish that board in 1982. That was a really bad move, so obviously bad that I won't bother saying "in my opinion".

@erich To the best of my knowledge, no U.S. aircraft manufacturer uses the SI system as a standard or publishes W&B info in metric as the standard. My guess would be that all other countries do. What Boeing does (or at least did up until I retired in 1999) is design and publish in the US measurement system. However, in the Boeing manuals I have, there is a kilogram page following each pound page for the convenience of SI users. This produces the oddity of inch-kilograms for moments. Also, for each 'gallon' page there is a following 'liter' page.

@PTwr Thanks for the info. I'll give that I try when I get back to working on it. Obviously web technology is not my strong suit!   LOL

@Terry But you do know planes a lot :) Glad to see you changed it already. I checked in IE 9, 10 and 11 and all seem to work fine now.

Putting cargo in the right place to maintain lateral CG position is easy, there is little place other than near the center to put it.
It's a different story with the fuel in the wing tanks, a wing with more fuel is heavier and will tend to drop down and requires the pilot to correct for that using the ailerons. This will introduce drag which will increase fuel consumption and lower the speed.
The limit described in the manual is usually in weight of fuel imbalance. And where the limit is depends from plane to plane and relies on the inherent roll stability and the roll control of the ailerons.

what percentage of total weight is fuel, typically?  understand that likely this varies by aircraft, so some sample upper and lower limits to that range would be helpful to the calculation.

Jet A is roughly 0.8 kg per litre, and a B747 holds something like 185,000 litres. That makes the fuel around 150,000 kg, or something like 30-35% of the total weight. A Cessna 172s's fuel is something like 150kg (210 litres of avgas at 0.7kg/litre), making it around 14% of the total weight. Both situations assume full fuel load at maximum takeoff weight

Lateral trim is indeed important. Although there are no "limits" prescribed for most light GA aircraft you can get a feel for how an imbalance can affect flight in some of them.
This is actually a fun exercise in a Cherokee:

Fill one tank to the tab (~16 Gallons, 96 pounds of fuel)
If you're solo this should be the right tank.
If you take an instructor the tank on the side of the lighter person.
Fill the other tank completely (~25 gallons, 150 pounds of fuel).
If you're solo fill the left tank.
If you take an instructor fill the tank on the side of the heavier person.
Start, run-up, take off, and climb out on the tank you filled to the tabs.
Fly around for 30-45 minutes to burn off some fuel.
With typical fuel burns you should wind up with 6-7 gallons, or around 40 pounds of fuel left in the low tank.  That's roughly a 100lb imbalance 3 feet out on the wing.
Turn into any wind, slow to around 80 knots, bring the ailerons to neutral, and wait a few seconds.

In most cases if the aircraft is rigged properly there will be a slight roll toward the full tank (heavy side). You will find you need to maintain some light aileron pressure in order to keep the wings level.
Remember to switch to the full tank after completing this little demonstration.
You don't want the engine to quit on your way home - that would be embarrassing!

A similar demonstration can be arranged with a Cessna 172, or really any aircraft that has a fuel selector which allows you to burn fuel from one wing exclusively on demand, but you should consult the POH and/or a qualified flight instructor about the particulars of the aircraft's fuel system before you go and try this on your own. I happen to know it's relatively safe in a PA-28, but I can't speak for the fuel systems on other aircraft.

The net effect of poor lateral trim is that the pilot (or autopilot) needs to maintain some control input to counteract the aircraft's tendency to roll toward the "heavy" wing.
Large aircraft (and "fancy" light aircraft like the Beechcraft Bonanza) have aileron trim to help out with this, but for aircraft without that particular luxury the pilot needs to hold the yoke/stick to maintain the desired control input.
As others have mentioned, larger aircraft often have valves and pumps to transfer fuel between wing and fuselage tanks to help achieve proper lateral trim.
The best most General Aviation aircraft can usually offer in this regard is a fuel selector that lets you burn from the left or right tanks: You burn fuel from the heavy wing until it's not heavy anymore.
Non-Fuel loads can be managed in a similar way: If they're liquid provide several storage tanks and pump them around as needed (there are gliders that do this with water ballast, though I believe that's primarily for pitch trim). If they're solid (or a liquid load in fixed containers), load them so they're well balanced, and secure them so they don't shift in flight.

The weight imbalance is serious. There is even a fuel balance valve in most jets and a warning for fuel imbalance. However, this valve played a significant role in the accident of Air Transat Flight 236.
As well, in some small passanger aircrafts, you might be weighted prior to boarding and seated so that the plane is balanced.

(I'm not a professional here. If a better answer appears, I may delete this one since it's not really detailed.)

You don't need to delete it, unless it's wrong or irrelevant: if a better answer comes in it will just get more votes :) your post answers the question so is perfectly acceptable on the site, even if lacking a little in derail or scope

@JonStory Well, if you think :) (btw, I've got about 40k rep so I do know how SE works ;) )

Hey, you could have just made one REALLY good answer that got 200 rep a day for like 200 days :p


text:
How are military aircraft type certificated?
It seems that FAA requirements for any particular aircraft revolve heavily around the aircraft's type certificate.  Civil aircraft are broken up into several categories based on weight or passenger count, etc.  But I did not see a section on military operations.
Do military aircraft undergo type certification?

Military aircraft are mentioned here.

good information, but i only see military surplus aircraft in that list.  does that mean non-surplus military aircraft don't get type-certificated?

Let me get this straight.  Necktie wearing bureaucrats are going to demand many nitpicky things from airplanes and helicopters that carry big cannon, bombs and missiles

I know in American English any noun may be verbed, but there's no need for "certificated" when there is a better, more-established word: "certified".

"Type certificated" is the common usage in American aviation circles to describe an aircraft that has undergone type certification. See e.g. usage of the phrase by the FAA here and here.

In the US, the military is not required to obtain FAA type certificates for their aircraft; the FAA is not the authority certifying a military-registered plane's airworthiness. They have the option of accepting an FAA type certificate (which they tend to do on aircraft based on civilian ones), but they run their own certification agencies for airworthiness, which are all that matters if the plane is in the military registration system. Here's an Air Force policy on the matter, here's a Lockheed powerpoint on the option to use FAA certification on commercial derivatives.

"In the US, the military is not required to obtain FAA type certificates for their aircraft"

That's not entirely true, per AFPD 62-6 Section 2.2.7, USAF must obtain and maintain FAA certification for civil aircraft acquired or modified by the Air Force if the primary mission for the aircraft is the transport of passengers. For all other civil aircraft acquired or modified, USAF shall obtain and maintain FAA certification to the maximum extent practical.

@fergbrain Thanks. I’ll track down the other DoD standards tonight to see of there are other service policies requiring FAA approval in various cases and update the answer.

Start with FAA Order 8110.101, which is for Military Commercial Derivative Aircraft. It's essentially the cross-domain authority (between civil and military airworthiness authorities) and defines the "seam" between them. Also check out MIL-HDBK-516.


text:
Are the panels in this vertical stabilizer part of the flight controls?
I've noticed some sort of "flight controls" on an A319 vertical stabilizer.
Here's the original photo

What are those things? Are they used for controlling the movement of the aircraft?
Source: Airliners.net (modified to indicate the segments of the vertical stab).

The part I have outlined below is the rudder. That is the only control surface on the vertical stabilizer.
The segments on the leading edge are just the leading edge surfaces. One may include an antenna, but they are only segmented for easier manufacturing and assembly.
The ones just forward of the rudder are panels that cover the rudder attachment and actuating systems. They can be removed for inspection or maintenance purposes.
The rest of the surface, in the middle, is the main structure of the vertical stabilizer. There is a spar on the forward and aft sides, and ribs that run between them. This is all covered by one piece of skin. This creates a structure that is both strong and light weight.

Below is a cutaway showing the internal structure.


text:
What may cause a strong smell of paint thinner like odor and no power to master in a Piper Warrior?
I am sitting in the hangar with my Piper Warrior II sitting outside.  The battery is completely flat and the cabin smells like paint thinner.  This is the first time that this has occurred and if anyone has experienced this before please let me know what is going on!  This is a maintenance question and I am concerned leaving the plane in the current condition over night.  This is NOT a 100LL smell.

Is the battery dead, or dry? Did you check the fuel for proper 100LL and not Jet-A?

Is there any evidence that the battery has cracked and leaked fluid, causing the smell?

I would not consider the smell of battery acid to be close to that of paint thinner, but very much an acid smell without the pungency of white vinegar. The only volatile fluid I can think of would be the compass fluid in the mag compass.

For troubleshooting, the following would be helpful: local temperature conditions; hanger heated or not; frequency of aircraft use including when last used and by whom; any recent maintenance.

For battery troubleshooting, I would start by checking battery condition, fluid level and connections, checking voltage across the battery, checking voltage at the master solenoid, and switch.

I appreciate all of your input as this is something that I do not want to mess around with (fly the plane by jump start in these conditions).  Compass is fine as far as fluid levels, no evidence of leakage from battery (which is odd), connections appear good, battery is dead as fluid is still in it.  I am at a loss and I apologize for posting this type of question, but this is a very odd instance for me!  I am calling in to request an A&P to look at it tomorrow.  I will follow up with the fault if one is found (if it would help others).

Is there a logical reason for the battery being flat or are fuses/CB off? If not you may suspect it had an internal short circuit which heated the electrolyte and a reaction occurred, or there was a short circuit close to the battery (before any fuse/CB) and the odor is heated insulator.

Just got a call back today.  Battery was replaced under warranty.  Still not sure why it was ran down to ~4 volts.  Smell is 1000LL that had soaked into the cabin carpet around the fuel selector.  Still no clue why battery was ran down and why fuel is leaking!  Oh well, we will know more next week.

You've got two unrelated problems: The battery and the smell.
The Battery
Battery electrolyte doesn't really have a "smell" to it - at least not one you're going to notice in the cabin easily. If you open the cell caps on a newly-topped-off flooded battery and sniff you'll get a feeling for what the electrolyte smells like.
If you have a sealed battery you're not going to smell electrolyte unless the battery completely loses its mind and vents a large quantity of electrolyte - you'd definitely notice the mess in the battery box if that happened.  You may notice it in a conventional flooded-cell battery if the battery is overcharged (from high bus voltage, or from trying to charge a flat battery after jump-starting the plane), but again it's not going to be readily apparent in the cabin unless you've lost a large quantity of electrolyte or you're really boiling the battery from an over-charge.
If your battery's cells are full (or it's a sealed battery) and you didn't leave the master switch on then the likely cause of it being flat is that it's old and can't handle the low temperatures of winter anymore (or if it's a new battery it may have developed an internal fault and discharged itself).
If you left the master switch on putting the battery on a charger/conditioner overnight can usually bring it back to good operating condition. You can try that on a flat battery from an unknown cause too: The good charger/conditioners will warn you if they think the battery has an internal fault. If an overnight charge doesn't resolve the issue then you're going to need to replace the battery before further flight (and frankly replacing the battery is cheap insurance against getting stranded somewhere, so it merits consideration anyway).

The Smell
The smell of 100LL changes as it evaporates - lighter aromatic components vaporize faster, and will waft out of the cabin more quickly, leaving the smell from the heavier hydrocarbons lingering.
As you discovered avgas that's been evaporating for a while loses the characteristic benzene/toluene scent we're familiar with from sumping the tanks and has the general odor of turpentine/conventional mineral spirits ("paint thinner").
The first thing to check in the PA28 when you smell something like this in the cabin is the fuel selector valve, as it's a common failure point particularly in cold weather: The packing/tapered plug in the fuel selector valve has shrunk in the cold, and you're leaking fuel around the shaft (which is then soaking into the carpet and evaporating off, giving you the smell in the cabin). Other aircraft may have similar failure modes in their fuel selectors, but I'm a Piper guy so I can really only speak to their systems and associated quirks :-)
Since your shop identified the fuel selector as the problem it's an easy fix: Rebuild or replace the fuel valve and the leak will stop, probably for many years.  (Piper Service Bulletin 355 is relevant to this issue for older Pipers - the O-ring identified in the exploded diagram dries out and shrinks/cracks, allowing avgas to leak around the shaft. There may be something similar in Piper's service documents for the Warrior II, I'm not sure if it uses the same internal valve or not)
Another common failure point would be the O-rings around the primer plunger in aircraft with a mechanical primer (anything before the Warrior III).
This doesn't normally manifest as a lingering smell in the cabin but rather a smell when you operate the primer plunger (usually also accompanied by the plunger being stiff/hard to operate). Much like the fuel selector it tends to manifest when the weather gets cold and things shrink or become brittle, and the cure is to disassemble and appropriately service the plunger mechanism (replace the O-rings and re-lube the mechanism with a fuel-safe lubricant).

voretaq7♦ I am forwarding this to my A&P now.  This appears to be bang on with what is going on with my aircraft.  The only unanswered question would be what is draining the battery!  This was a new battery (only a few months old) and all electronics were switched off after my last flight (first thing I check in dead battery situations in all vehicles).  Any suggestions on what to look for/probe out?

FOUND ISSUE(s).  Battery was at four volts.  We think it is a voltage leak most likely coming from the removable GPS mounted in the dash.  The second issue was a fuel line fitting (one that connects to firewall) that was loose.  Fuel was leaking from the firewall down to the carpet.  Thank you everyone for your help.  Hopefully this post will help others with similar issues.


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How does one deal with a UNICOM operator directing aircraft like at a towered airport?
Seems like an odd scenario, however I have experienced this a few times now.  A non-towered airport will have someone on the radio directing traffic and reading out weather info like barometric pressure.
I find this to be extremely dangerous and unwanted as the times I have encountered this, the person on the radio has no reason or training to do this (among other things).
Question:  If faced with this scenario what would you do?  Would you report the airport?  Am I looking too much into this and should just let it go?

Sounds like a UNICOM operator on shared CTAF frequency. This is usually considered a service to pilots but it can also cause conflicts as evidenced by this article from Flight Training magazine.

There was recently a push to introduce this in Australia, and I'm pretty sure they relented and it's on a trial somewhere. Personally I am uneasy about the prospect of someone acting like an ATC without any ATC training! I think it is easy enough to deduce the conditions and traffic flow without this 'service'.

It's probably on the late side now, but a good addition to this question would have been a specific example of the phraseology used by the operator. For example, there's a huge difference between, say, "cleared to land runway 19" and "runway in use is 19".

The UNICOM operator appears to be doing his/her job. You can always not fly to that airport and go somewhere else where there is only CTAF.

You're almost certainly talking about a UNICOM operator, who is just an airport (FBO) employee who gives information to pilots on their request. The FAA's Pilot Handbook says:

UNICOM is a nongovernment air/ground radio communication station which
  may provide airport information at public use airports where there is
  no tower or FSS. On pilot request, UNICOM stations may provide pilots
  with weather information, wind direction, the recommended runway, or
  other necessary information. If the UNICOM frequency is designated as
  the CTAF, it will be identified in appropriate aeronautical
  publications.

The important point is that UNICOM is not ATC. A UNICOM operator cannot give you instructions, clearances or require you to do anything. He can only give you information, and as PIC it's up to you to decide how to use that information.
The article that Porcupine911 mentioned is a great example of that. A student pilot thought that UNICOM was telling him which runway to land on, whereas in fact the operator was only recommending a runway (presumably based on the prevailing wind or whatever). Unfortunately, the operator recommended the opposite runway to the one that was actually in use by other pilots.
Like pilots, UNICOM operators vary in their experience and competence. A good UNICOM operator (in my opinion) will give you the local wind and altimeter information, tell you if there are other aircraft in the pattern or nearby, tell you which runway is in use by those aircraft, and give you any essential safety information (like a closed runway). But as PIC, I'm still going to listen to CTAF and AWOS and look for other traffic before choosing a runway and joining the pattern. It's also common for no one to reply to a UNICOM call (because the operator is out fueling aircraft in his other role as lineman, for example), in which case you're going to have to sort things out without it anyway.
So there's no need to "report" anything: UNICOM operators are a well-known and official service, but you have to know how to use them and what their limitations are.

Wonderful information and very appreciative for the followup.

Reference 47 CFR 87.213. Attempting to direct air traffic via Unicom is illegal, I would report that.

(c) Unicoms must not be used for air traffic control (ATC) purposes other than to relay ATC information between the pilot and air traffic controller. Relaying of ATC information is limited to the following:

Revisions of proposed departure time;
Takeoff, arrival or flight plan cancellation time;
ATC clearances, provided a letter of agreement is obtained from the FAA by the licensee of the unicom.

47 CFR 87.213 is an FCC regulation, so this should be reported to the FCC, not the FAA.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

@Sean I beg to differ. It cites the appropriate regulation and states the recommended action. It could be improved by adding a link to the regulation and formatting a bit but I’ve seen far worse first posts. So welcome to Aviation.SE Spencer!

I feel like this answer is missing an important piece of information, though: who should this be reported to? The FAA, the FCC, or someone else?

@tanner swett, I agree this could be helpful to include, but it wasn’t asked in the original question.  And most pilots would know that a violation of the CFRs would be reported to the FAA.  The FCC would only have jurisdiction if the UNICOM station does not have a radio station license.

@sean, I disagree.  This was a perfectly succinct answer to the original question as asked.

Let me clarify a point...  most pilots probably wouldn’t rat out a UNICOM operator to the FAA.  At least not right away...  A frank and open conversation should happen though, if any pilot feels the operator stepped into an ATC role.

@MichaelHall "And most pilots would know that a violation of the CFRs would be reported to the FAA." - I think you just proved my point, because the regulation mentioned in the answer is an FCC regulation, not an FAA regulation. So clearly it's necessary to state who the violation should be reported to; if you simply state "I would report that" without saying who to report it to, then that fails to answer the question as asked.

As suggested by J. Hougaard, I edited this answer to say that this should be reported to the FCC, not to the FAA. If anyone disagrees with this edit, please feel free to roll it back.

@tanner swett, I concede on your first point that this is an FCC violation.  Bad on me for not checking.  I presumed their main focus was frequency allocation and station licensing.  Secondly, to clarify "I would report that":  I would never make a formal report to the FCC, FAA, FBI, or any federal authority without first attempting to resolve the issue myself.  If I thought the Unicom operator was overstepping his or her bounds and directing traffic I might correct it on the radio, otherwise I would seek them out for a friendly informative discussion about limits and liability.